The LLVM developers seem to be driven to replace all parts of the GCC toolchain and libraries with home-grown alternatives under BSD-style licenses. The latest addition to the project is libc++, an implementation of the C++ standard library which is faster and uses less memory than the GCC libstdc++. The developers also intend to support standard library debugging which is ABI compatible to the release version, which should help developers cut down on lengthy recompile-and-debug cycles. The project is still in an early state but it already implements 85% of the C++0x standard library. As with the rest of the LLVM project, the development of libc++ is being supported by Apple.

Apple takes the code, modifies it a few ways, and rebrands it the iComplier, which they begin charging hundreds of dollars for. They stop contributing back to LLVM at all.

Very possible scenario, but how would this hurt the non-apple community? The code up until apple's fork would still be open and up for grabs. It is nice to have Apple contributing to LLVM up till this point, but it isn't a must.

The modifications to the iCompiler mean that the output can only run on OSX/Apple hardware, and they simultaneously modify OSX so that it will only run software compiled by the iCompiler.

Again, this would not affect any other OS than Apple's. Even if a BSD licensed LLVM would not exist, there would be nothing stopping Apple from doing this. Either way, this will only affect apple's OS....